Gmail has a nice feature:
You can delegate access to your Gmail to another person so they can read, send, and delete messages on your behalf. For example, you can delegate e-mail rights to an admin in your organization, or you could delegate your personal email access to your spouse. The delegate can also access the other person’s contacts by clicking the Contacts link. Clicking the To,Cc, or Bcc links in the mail compose window will also bring up your contacts.
You won’t be able to give anyone permission to change your account password or account settings, or chat on your behalf. You can specify up to 10 users. Google Apps for Business, Education, and Government customers can specify up to 25 users. (source & details)
What happens when you get to 10 emails for which you have access? You can’t add other ones anymore.
How to solve this?
Let’s say that emails firstname.lastname@example.org, email@example.com, firstname.lastname@example.org … email@example.com all give access to firstname.lastname@example.org. If you want to add email@example.com, you can’t. The solution is to login as firstname.lastname@example.org (or any other one in that list) and go to Settings => Accounts, and in there remove the granted access. In this way, you will be able to add email@example.com, replacing the missing address.
Unlike granting access, which takes a while to process, removing the granted access is instantly done.